Loop detector systems detect the change in inductance of a wire loop that is placed in the ground that occurs when a vehicle or other large object enters that loop. The wire loop is often driven by an AC electric current which is provided at or near base a frequency set by an oscillator.
Loop detectors have been used in a variety of circumstances. For instance, loop detectors have been used at traffic lights to indicate that a vehicle is present so that the traffic light can be changed and the vehicle can proceed through an intersection. In another example, loop detectors have been placed in front of a barrier such as a gate. When a vehicle enters the loop, the detector indicates the presence of the vehicle to a moveable barrier operator, which moves the gate so the vehicle can pass. Similarly, the gate is held open until the vehicle has left the pathway of the gate. The above-mentioned systems suffered from the disadvantage that the identity of the vehicle or occupant was never determined.
To identify the occupant, other systems used RF identification tags to identify a vehicle when the vehicle was in the vicinity of the barrier to be opened. In these systems, the RF Identification tags had information stored on them. An antenna near the barrier directed a signal at the tags to read the information. In this way, the identity of a user (written into the tags) was retrieved at the entrance of a barrier. Once the RF ID was retrieved and matched with the IDs of users who could proceed through the barrier, the barrier was opened. Similarly, other previous barrier control systems detected an RF transmission manually generated by a user at a transmitter.
However, even though these systems automatically opened barriers and considered the identity of the user in making determinations to open a barrier, these previous systems also suffered from certain shortcomings. For example, the cost of special antennas and processing circuitry for reading the RFID can be high and beyond the reach of many private users. In addition, in many environments, radio interference is a problem, making sensing the RF tags difficult or leading to errors in processing the information. Also, the sending of the user generated code does not provide a sufficiently automatic operation.